1. Field of the Invention
The present invention generally relates to lasers and, more particularly, to a new laser construction that produces an output power pulse that greatly exceeds the previous optimal power of the prior art.
2. Background Description
Lasers consist of an optical cavity that provides feedback and a gain medium that provides optical amplification. Previously, lasers have used mirrors (dielectric or metallic) to build the optical cavity, such as a Fabry-Perot oscillator. Unfortunately, these mirrors have a fixed reflectivity. This necessitates choosing the reflectivity of the mirror to balance the amount of output coupling against the amount of feedback.
Losses in a laser can be grouped into two different categories: a) non-useful losses due to diffraction, scattering, and absorption; and b) useful loss due to coupling of output power through a partially transmissive mirror. It is obvious that optical output power is maximized by minimizing the first group of losses. However, selecting a reflectivity that optimizes output is more complicated. At first it would appear that a mirror reflectivity of one would be desirable. Although this maximizes optical feedback, amplification, and energy build up in the cavity, it also reduces optical output to zero. Recall that in the absence of absorption, the mirror reflectivity and transmitivity must sum to one (conservation of energy).
T=1xe2x88x92Rxe2x80x83xe2x80x83(1)
where T is transmitivity and R is reflectivity.
Reducing the optical reflectivity to zero is also sub-optimal as it reduces feedback, energy build up in the cavity and the benefit of multiple passes through the gain medium is lost. The result is low optical output and the laser does not lase. Currently, the reflectivity must be carefully selected to achieve maximum output power. This optimal reflectivity must balance the benefits of lower reflectivity (high percentage output, low gain) against higher reflectivity (lower percentage output, higher gain).
The maximum output power is obtained when
T=xe2x88x92L+{square root over (gLi+L )},xe2x80x83xe2x80x83(2)
where T is the mirror transmitivity, g is the unsaturated gain per pass, and Li is the non-useful fraction of intensity lost per pass. Li is given by
Li=Lxe2x88x92T,xe2x80x83xe2x80x83(3)
where L is the total loss per pass given by
L=1xe2x88x92R1R2exe2x88x92al,xe2x80x83xe2x80x83(4)
where R1 is the reflectivity of the first mirror, R2 is the reflectivity of the second mirror, xcex1 is the absorption loss per unit length, and l is the length of the cavity. The optimal power output is given by
P=IA({square root over (g)}xe2x88x92{square root over (Li+L )})2,xe2x80x83xe2x80x83(5)
where I is the saturation intensity, and A is the cross sectional cavity area.
Q switching is a widely used method to create much larger pulses than usual laser operation allows. This method increases the population inversion inside the laser cavity either by A) removing the cavity feedback or B) increasing the cavity losses. While in this low Q state normal pumping increases the population inversion inside the cavity. When the population inversion has reached the desired levels the cavity feedback is restored or the source of increased loss is removed. Next, the laser begins to oscillate and the energy stored in the population inversion is converted into photons resulting in a high energy pulse. This pulse rapidly depletes the stored population inversion. Typical pulses are only a few tens of nanoseconds long.
Q-switched lasers have many practical applications where brief but intense pulses are required, including: range finding, cutting, drilling, and nonlinear optical studies.
Specific methods of Q-switching include:
1. Rotating mirrorsxe2x80x94a mirror or 90 degree prism is mounted to a motor shaft and spun so that it is only aligned with the other cavity mirror for a brief time. This method suffers from the slow speed of mechanical devices.
2. Electro-opticxe2x80x94a crystal that becomes birefringent upon application of a voltage is placed in the laser cavity along with polarizing elements. This is the fastest method and can achieve pulses of less than 10 nanoseconds. Disadvantages of this method include: it requires a high speed, high voltage electrical pulse source; these intense electrical pulses can produce severe electrical interference in nearby equipment; and the optical crystal and polarizers are optically lossy even in the high Q state and are subject to optical damage.
3. Acoustoopticxe2x80x94an acousto optic modulator is used to diffract light out of the cavity for the low Q state. These modulators have low insertion losses. However, this method is limited to kHz repetition rates.
4. Passive saturable absorberxe2x80x94an absorbing medium that is easily saturated is used to provide the increased loss. When the population inversion builds up enough that the gain exceeds the saturated loss the laser lases, depleting the population inversion. An organic dye is often used as the saturable absorber.
5. Thin film saturable absorberxe2x80x94a thin metallic film is used as the absorber until light in the laser cavity evaporates or burns it away. This is a one shot type of method.
The rate equation analysis can be depicted as:
xe2x80x83dn/dt=KnNxe2x88x92Ycn, and
dN/dt=Rpxe2x88x92N/T2xe2x88x922*KnN,
where n(t) is the cavity photon number, N(t) is the inverted population difference, Yc=1Tc is the cavity decay rate, Rp is the pumping rate, T2 is the excited electron lifetime, the reciprocal of the decay rate, and K is the coupling coefficient between atoms and photons. 2* is a constant between 2 and 1. This constant is 2 for lasers with only two transition levels or for multilevel system where the lower transition level has a long lifetime. Siegman refers to these as xe2x80x9cbottle neckedxe2x80x9d systems. The constant is 1 for systems that have very short lifetimes for the lower level transition.
During the pumping interval the equations can be simplified. No oscillation occurs, so n=0 during this period, reducing the second rate equation to
dN/dt=Rpxe2x88x92N/T2,
which has a solution N(t)=RpT2{1xe2x88x92exe2x88x92t/t2}. Thus, the inverted population approaches a maximum value of RpT2 exponentially. This value RpT2 is also the maximum number of photons that can be produced in the pulse when it is switched. Generally for proper operation the gain medium must have a reasonably long lifetime in the excited state. Note that Q switching is not practical for most visible gas lasers or organic dye lasers as the population lifetimes are a few nanoseconds. The time over which a population can be built up and stored is just too short for a pulse to develop.
It is therefore an object of the present invention to provide a variable reflectivity mirror to increase the amount of available laser output power.
It is also an object to provide high speed optical pulses from a laser.
According to the invention, output for a laser is greatly increased by altering the transmitivity of a superconductor layer which serves as one of the mirrors of the laser cavity. The superconductor layer is switched between a superconductive state, having a high reflectivity, and a non-superconductive state, having a lower reflectivity. When the mirror is in its superconducting state, power in the laser cavity will reach a high level and light output will be low, and when the mirror is in its non-superconducting state, power output will be high and the cavity power will be lowered.